The invention relates to F-type railroad car couplers, especially the rotary-type in which a pinhole is formed in the butt end of the shank of the coupler. Existing F couplers have a pinhole with a backwall, i.e. the wall closest the butt end of the shank, which comprises a small elliptical or hexagonal-shaped cylindrical pin bearing surface from which curved top and bottom portions of the backwall extend and diverge in the direction of the butt end of the shank, so that the pin bearing surface projects into the pinhole. The constant movement of the pivot pin against the backwall of the pinhole, when the coupler is in a normal horizontal pull position, wears away this projection or crown so that the backwall is vertically straight or cylindrical between opposing ends of the pinhole. It can be readily appreciated that, upon vertical angling of the coupler, the pivot pin will then alternately bear against portions of the backwall adjacent opposing ends of the pinhole to produce at these points, large concentrations of stress which can cause fatigue and eventual fracturing of the shank in the area of the pinhole. The invention is directed to a pinhole with a highly improved pin bearing surface.
Briefly stated, the invention is in a railroad car coupler having a pinhole with a specially configured backwall that has a crown or projection which is at least twice that of the aforementioned existing rotary F couplers. The backwall is provided with a cylindrical pin bearing surface which has a generally rectangular shape that has a pair of spaced, longitudinally extending curved marginal edges from which a pair of reversely curved surfaces extend rearwardly toward the butt end of the coupler shank. The convex-concave curvature of these curved surfaces are such that, upon one-quarter inch wear of the backwall, the height of the pin bearing surface in relation to overall height of the backwall will not be radically changed to a point where, upon vertical angling of the coupler, the pivot pin will contact the shank in such a way to produce the aforementioned, undesireable high stress concentrations.